The invention relates to high gain, low drift operational amplifiers having field effect transistor input stages, especially as used in high speed sample and hold circuits.
A type of analog-to-digital converter (ADC) known as a subranging ADC is one of three common types of high speed ADCs. ADCs of the successive approximation type are simple in structure, and may be very accurate, but they have very slow conversion times, due to the serial nature of the conversion processes therein. For example, for a successive approximation analog-to-digital converter with 12 bits of resolution, conversion times of about 0.6 microseconds to one microsecond are typical. At the other extreme, ADCs of the "flash converter" type have very short conversion times, requiring one cycle of operation. However, the high speed is achieved at the expense of greatly increased circuit complexity. Flash converters with 8 bits of resolution and conversion rates as high as 100 megahertz represent the limits of present integrated circuit technology. ADCs of the subranging type provide an intermediate compromise between flash encoders and successive approximation ADCs. The present state-of-the-art for subranging ADCs is thought to be represented by a 12 bit, 10 megahertz subranging analog-to-digital converter Model No. CAV-1210, manufactured by Analog Devices Corporation. Subranging analog-to-digital converters typically use a sample and hold or track and hold circuit that produces a sample voltage, which is encoded by a MSB (most significant bit) flash encoder to produce an MSB word.
The sample and hold circuits required in subranging analog-to-digital converters must be very accurate. Typically, such sample and hold (or track and hold) circuits include a switching diode sampling bridge that is isolated from the analog input signal by a high speed, highly accurate open loop input buffer. A sampling capacitor is connected to the output of the diode sampling bridge, which is actuated in response to a "sample command", and is applied as an input to a second high speed buffer. Typically, the output impedance of a sample and hold circuit of the type used in subranging analog-to-digital converters, such as the HTS0010 track and hold circuit made by Analog Devices Corporation, is about 5 ohms. The gain of the track and hold circuit is adjusted by an external potentiometer.
Although use of feedback amplifiers to achieve high input impedance and low output impedance is a common expedient, up to now no operational amplifier has been known having the very low input offset voltage, the high degree of temperature stability, the high input impedance, and the high bandwidth that would be needed to allow use of a closed loop output stage for a sample and hold (or track and hold) circuit that would be suitable for a 12 bit, 10 megahertz subranging analog-to-digital converter.